The present invention relates to the technology of setting up frequency bands used by base stations that communicate with wireless terminals. More particularly, the present invention relates to the technology of setting up frequency bands for a plurality of base stations in a system where a plurality of base stations in communication with a plurality of wireless terminals are connected to networks such as wired LANS.
In a system where a plurality of base stations are provided, radio signal interference sometimes causes communication errors. Interference may be caused by radio waves from other base stations, by radio waves from wireless communication terminals connected to other base stations, or by external noises independent of a wireless LAN system.
Today, three spread spectrum communication methods are in common use for wireless LAN communication: direct sequence (hereinafter called DS), frequency hopping (hereinafter called FH), and a combination of both. In the DS method, a base station (for example, a wireless communication hub) spreads the frequency by transferring a signal to which a code, which indicates a specific frequency bandwidth occupied by the base station, is attached. In the FH method, each hub transfers signals while changing the frequency at a regular interval. Although both methods are designed to be less susceptible to noises, the DS method that occupies a specific frequency bandwidth for data transmission transfers signals more speedily than the FH method that transfers each unit of data across a specific frequency bandwidth.
In practice, about 20 wireless terminals may be connected to one wireless hub. Therefore, many wireless terminals require many base stations. When installing a plurality of base stations in the same area, an arrangement and a method that eliminate interference among base stations are required.
An earlier patent disclosure dealing with this is found, for example, in JP-A-9-275401. In this example, the master hub, one of the plurality of wireless hubs, specifies frequency bandwidths to be used by other hubs to minimize interference among the coverage areas of wireless LAN systems each using the FH method. This technology, specifically designed for the FH method, is not applicable to the DS method that occupies a particular bandwidth.
The DS method, which allows one of the base stations to occupy a particular frequency bandwidth, does not allow two base stations to share the same bandwidth when the coverage areas of a plurality of base stations overlap. Therefore, the system must prevent interference in one of two ways: (1) prevent the coverage area 123A of a base station 121A on a wired LAN 120 from overlapping the coverage area 123B of a base station 121B on the wired LAN as shown in FIG. 11 and (2) separate the frequency bandwidth of the base station 121A and that of the base station 121B.
Currently, the set-up task such as the allocation of frequencies to base stations is performed manually. Therefore, as more base stations are used in the as same area, it becomes more difficult to implement the optimum setup because interference among base stations must be considered and, at the same time, the setup task takes long. More base stations in the same area result in more wireless terminals connected to the base stations. An increase in the number of wireless terminals makes the setup task even more difficult. In addition, unexpected external noises, as well as interference caused by wireless terminals used by moving users, must be considered.